Scientific research and practice of sports selection confirm the importance of peak height velocity (PHV) and maturity offset (MO) for the development of physical fitness and training programs for young athletes. This requires a method for assessing these indicators that combines maximum validity, ease of use, and confirmed results of testing in wide groups of athletes. In this paper, we aimed to test the interchangeability of two empirical equations for assessing maturity: Mirwald’s equation, created in 2002 and requiring measurements of standing height, body weight, sitting height, and leg length, and Moore’s equation, introduced in 2015 and requiring only standing height to be measured. The study involved 314 healthy non-athletic boys aged 13 to 17 years and 403 football players aged 8 to 18 years. Athletes were divided into 3 age groups: school childhood, puberty, and adolescence. All volunteers had their standing height, sitting height, leg length, and body weight measured. Based on the obtained data, peak height velocity (PHV) and maturity offset were calculated using Mirwald’s equation and Moore’s equation. Bland-Altman analysis was used to assess the consistency of these maturation parameters. The use of the simpler Moore’s equation leads to underestimation of MO and overestimation of age of PHV by an average of 2 years in subjects from all groups. The maximum error (3 years) of this method was found in adolescents aged 15 to 17 years. Replacing Mirwald’s equation with Moore’s equation for the purpose of simplification may lead to false positive predictions regarding age of PHV and overestimation of physical capabilities in a given athlete.